1. Field of the Invention
The present invention relates to an equalizer and equalization method for equalization for avoiding characteristic deterioration due to symbol synchronization errors between transmitter and receiver, which is often a problem arising in mobile communications, etc.
2. Description of the Related Art
To respond to increasing demands of mobile communications using a limited frequency band, it is necessary to increase frequency utilization efficiency. An example of the most efficient means of this is QAM (Quadrature Amplitude Modulation).
Using the QAM for a mobile communication requires changes in the envelope and phase due to fading variations to be coped with. Examples of countermeasures are disclosed in “Transmission Path Distortion Compensation System” disclosed in Japanese Patent Publication No. 6-1908 and Reference 1: “16 QAM Fading Distortion Compensation System for Terrestrial Mobile Communications” (Sanpei, et al., Institute of Electronics, Information and Communication Engineers Collected Papers B-II, Vol. J72-B-II No.1 pp7–15, January 1989). This proposed system is a pilot symbol interpolation synchronous detection system that measures fading distortion from periodically inserted known frame symbols and estimates and compensates for fading distortion of all frame symbols by interpolating the time series. In this case, 16 QAM is often used as the target modulation system.
In association with the above-described proposed system, Reference 2: “16 QAM/TDMA-Based Symbol Timing Reproduction System” (Sanpei, et al., TECHNICAL REPORT of Institute of Electronics, Information and Communication Engineers (RCS92-106 (1993-01)) is proposed. The MAM (Maximum Amplitude Method) used here is a method of using a sample showing a maximum amplitude as a synchronization point. Provided that a known frame symbol has a maximum amplitude, a simulation result confirms that if the frame length is a few tens of symbols and the oversampling number is 32 times, the MAM can obtain a satisfactory characteristic.
Here, a modulation/demodulation apparatus for carrying out above-described fading distortion compensation will be explained. FIG. 6 is a block diagram showing a configuration example of a conventional modulation/demodulation apparatus. As shown in FIG. 6, the transmitter is constructed of a pilot symbol adding section 101, amapping section 102, aquadrature modulation section 103, abaseband-RF conversion section 104 and an antenna 105. On the other hand, the receiver is constructed of an antenna 106, an RF-baseband conversion section 107, a quadrature detection section 108, a synchronization processing section 109, a pilot symbol distortion measuring section 110, a symbol data distortion compensation section 111 and a de-mapping section 112.
First, an operation of the transmitter will be explained. The pilot symbol adding section 101 adds pilot symbols to an input digital signal string and outputs the result to the mapping section 102 as a pilot symbol insertion signal string.
The mapping section 102 carries out mapping processing on the pilot symbol insertion signal string according to a mapping table which is common to the transmitter and receiver and outputs the result to the quadrature modulation section 103 as an I-phase mapping signal and Q-phase mapping signal.
The quadrature modulation section 103 carries out quadrature modulation processing using the I-phase mapping signal and Q-phase mapping signal and outputs the result to the baseband-RF conversion section 104 as a baseband modulation signal.
The baseband-RF conversion section 104 converts the baseband modulation signal to an RF modulation signal and outputs to the antenna 105. The antenna 105 outputs the RF modulation signal to a radio communication path.
Then, an operation of the receiver will be explained. The RF-baseband conversion section 107 converts the RF modulation signal received by the antenna 106 to a baseband modulation signal and outputs to the quadrature detection section 108.
The quadrature detection section 108 carries out quadrature detection processing on the baseband modulation signal and outputs the result to the synchronization processing section 109 as a quadrature detection I-phase signal and quadrature detection Q-phase signal.
The synchronization processing section 109 detects frame timings from the quadrature detection I-phase signal and quadrature detection Q-phase signal using synchronization timing detecting means such as an MAM and outputs the result to the pilot symbol distortion measuring section 110 and symbol data distortion compensation section 111 as an I-phase frame signal and Q-phase frame signal.
The pilot symbol distortion measuring section 110 measures the amount of distortion of pilot symbols from the I-phase frame signal and Q-phase frame signal and outputs the result to the symbol data distortion compensation section 111.
The symbol data distortion compensation section 111 compensates the I-phase frame signal and Q-phase frame signal for distortion based on the distortion measured value and outputs the result to the de-mapping section 112 as the I-phase distortion compensated signal and Q-phase distortion compensated signal.
The de-mapping section 112 carries out de-mapping processing on the I-phase distortion compensated signal and Q-phase distortion compensated signal according to a mapping table which is common to the transmitter and receiver and outputs the result to the outside as an output digital signal string.
While the conventional modulation/demodulation apparatus has effects on fading distortion, it has a defect that compensation for distortion specific to radio unit is not considered. When the oversampling number is large, the amount of calculations becomes enormous as the communication speed increases, which is a significant problem in design. Reducing the oversampling number to solve this problem will cause another problem of producing a floor error due to jitter.